Dodge Durango Engine Temp Gauge Goes Full Hot At Times

Michael Mobile Technician Clinton, Utah Posted   Latest  
Case Study
Electrical
Emissions
2003 Dodge Durango SLT 4.7L (N) 4-spd (46RE)
P0118 - Engine Coolant Temperature Sensor 1 Circuit High
Temperature Plausibility

A shop called me in to help with a pesky Dodge Durango. The customer complaint was that at times the temperature gauge goes hot. The shop had over the last 6 months replaced the sensor with first aftermarket then factory sensors with no resolution. Freeze frame data showed -41F at time of code setting. The assumption is that the gauge goes full hot in an open or short circuit to make sure the vehicle gets serviced. So the question then was, is it a connection, wiring or PCM problem.

In order to remove the sensor from the equation, I backprobed the ECT connector and used a decade box to simulate the thermistor contained in the sensor. At first it was quite confusing. With the sensor open the temp light inside went to full hot. The scan tool temp reading barely flinched. With very low resistance the result was the same. I switched to the DRBIII emulator tool in case the problem was with the aftermarket tool. Same readings. After looking over the diagrams of the system it was confirmed that the same sensor addressed the gauge and the PCM inputs. Then I noticed that the sensor voltage was changing drastically.

At 400K Ohms the voltage read 4.97 Volts. In this case the value seemed to be in range because it now read -43.6F. This was pretty much the limit where the temp worked correctly. When the resistance was increased, the voltage changed but the temp value appeared to be a substituted value. At 40K Ohms the temperature read 24.8F. At 400 Ohms the temp read 246F. Fewer Ohms than that and back to the substituted values.

Since the problem was intermittent, I decided to look for wire rubs or any indicator that would point to an open. I saw in the service information a splice pack for most sensor grounds. There was nothing disturbed with the loom. I then started to do a wiggle test with the scan tool screen in front of me. I grabbed the pcm wires just below the connector and gently moved the wires. And there it was. Voltage went from around 2.5V to 5.01V. Wiggle a little more and the value went back. In drag testing the terminal barrels, they all tested good. The problem is most likely a bad solder joint within the PCM. Luckily we were able to find a PCM at a local wrecking yard.

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Helpful
Steven Instructor
Cumming, Georgia
Steven Default
 

MICHAEL, Excellent post! Your chosen diag process seemed ideal for the case. Where did you pick up the aftermarket resistance box? BTW: I use to employ a wiggle test with a Tech 1 connected to PRO COM (RS232 communication software). I would bring in the PIDS of interest on the Tech 1, which would simultaneously display on the monitor. Perform the wiggle and the PRO COM software would update…

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

The decade box was from Amazon. amazon​.​com/Elenco-Resista… It was inexpensive as compared to the Extech. I like the Extech better. This one does work fine though.

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Michael, I have an Extech for shop use and was thinking about the one above for my home work bench. What is it what you like better about the Extech? Thanks,

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

I guess I am old school. The Extech is heavier and seems more sturdy. The switches have a better feel to them. If you take care of it the cheaper one will work, I have had mine over a year and it seems OK. -Mike

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

As I understand the ckt; the PCM puts out a 5V signal and the sensor pulls it to ground, more so as it warms. So when the voltage goes up(2.5 to 5v) that would indicate an open outside of the PCM. An open internal would go to 0v, right? Unless the lead is shorting to a 5V source.

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Stephen, As you stated, if the circuit goes open then the computer reads 5 volts. The ground is shared by multiple sensors so that counts the sensor ground out. If the the pin feeding 5v to the sensor has become detached from circuit board like a broken solder joint that would give the same result as an open in the wire between PCM pins and sensor. The 5V supply is good inside. Since the…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Yeah, those substituted values can getcha. I'm sure you know, but for the sake of any future reader, if you switch to OBD2 ("generic") you won't get substitutions. I'm reading this late, my ISP spam-trap went crazy and took all the DN emails away. I thought the site had died.....LOL

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